Fix issue #5 when to few control points are given

package.json

@@ -1,7 +1,7 @@
 {
   "private": false,
   "name": "curve-interpolator",
-  "version": "2.0.4",
+  "version": "2.0.5",
   "description": "Interpolate values on a Cardinal/Catmull-Rom spline curve",
   "repository": "https://github.com/kjerandp/curve-interpolator",
   "bugs": {

src/core.ts

@@ -66,6 +66,16 @@ export function getPointAtT(t: number, points: Vector[], options: InterpolationO
   const [p0, p1, p2, p3] = getControlPoints(idx, points, closed);
 
   target = target || new Array(p0.length);
+
+  // handle special case that occurs when number of control points are less than 4
+  // and the last point is used to extrapolate the missing control points
+  if (nPoints === 3 && p0 == p1 && p1 === p2 && p2 === p3) {
+    for (let i = 0; i < p1.length; i++) {
+      target[i] = p1[i];
+    }
+    return target;
+  }
+
   for (let i = 0; i < p0.length; i++) {
     target[i] = func(weight, tension, p0[i], p1[i], p2[i], p3[i]);
   }

src/curve-interpolator.ts

@@ -25,9 +25,6 @@ import {
  * @param args input array
  */
 function extrapolateArgs(args:Vector[]) : Vector[] {
-  if (args.length < 4) {
-    args.unshift(args[0]);
-  }
   while (args.length < 4) {
     args.push(args[args.length - 1]);
   }

test/curve-interpolator.spec.ts

@@ -118,5 +118,19 @@ describe('curve-interpolator.ts', () => {
     expect(Object.keys(interp._cache).length).to.eq(0);
 
   });
+
+  it('should work with too few control points', () => {
+    const interp = new CurveInterpolator([
+      [888.48611, 481.364299],
+      [389.28611, 489.364299],
+      [389.28611, 258.964299],
+    ], { tension: 0 });
+
+    const closeToStart = interp.getPointAt(0.02);
+    const closeToEnd = interp.getPointAt(0.98);
+
+    expect(closeToStart[0]).to.be.lessThan(888.48611);
+    expect(closeToEnd[0]).to.eq(389.28611);
+  });
 });